To expose criminals who traffic animals, Rachel Nuwer went undercover—even posing as a prostitute.

A recent report by the World Wildlife Fund for Nature showed that between 1970 and 2014 the vertebrate population declined by an average of 60 percent. While this was mostly due to habitat loss, the illegal trade in wildlife—whether rhino horn, tiger bone, or animals captured for the exotic pet market—poses a growing threat to many species’ survival. But as National Geographic contributor Rachel Love Nuwer writes in her new book Poached: Inside the Dark World of Wildlife Trafficking, many brave individuals and organizations are battling to expose the criminals—and save the animals.